1. Field of the Invention
This invention relates to lithographic printing plates and their process of use. More particularly, this invention relates to lithographic printing plates which can be digitally imaged by infrared laser light.
2. Description of Related Art
Conventional lithographic printing plates typically have a radiation sensitive, oleophilic image layer coated over a hydrophilic underlayer. The plates are imaged by imagewise exposure to actinic radiation to produce imaged areas which are either soluble (positive working) or insoluble (negative working) in a developer liquid. During development of the imaged plate, the soluble areas are removed by the developer liquid from underlying hydrophilic surface areas to produce a finished plate with ink receptive oleophilic image areas separated by complimentary, fountain solution receptive hydrophilic areas. During printing, a fountain solution is applied to the imaged plate to wet the hydrophilic areas, so as to insure that only the oleophilic image areas will pick up ink for deposition on the paper stock as a printed image. Conventional lithographic printing plates typically have been imaged using ultraviolet radiation transmitted imagewise through a suitable litho film in contact with the surface of the printing plate.
With the advent of digitally controlled imaging systems using infrared lasers, printing plates which can be imaged thermally have been developed to address the emerging industry need. In such thermally imaged systems the radiation sensitive layer typically contains a dye or pigment which absorbs the incident infrared radiation and the absorbed energy initiates the thermal reaction to produce the image. However, each of these thermal imaging systems requires either a pre- or post-baking step to complete image formation, or blanket pre exposure to ultraviolet radiation to activate the layer.
Examples of radiation sensitive compositions and their use in making lithographic printing plates are disclosed in U.S. Pat. Nos. 4,708,925; 5,085,972; 5,286,612; 5,372,915; 5,441,850; 5,491,046; 5,340,699; and 5,466,557. European Patent Application 0 672 954 A2; and PCT/GB95/02774.
U.S. Pat. No. 5,372,915 is an example of a printing plate containing a radiation sensitive composition which is comprised of a resole resin, a novolac resin, a latent Broensted acid and an infrared absorber. In the preparation of a lithographic printing plate, the radiation sensitive composition is imagewise exposed to activating infrared radiation and the exposed areas of the printing plate are removed with an aqueous alkaline developing solution. Related U.S. Pat. No. 5,340,699 discloses the preparation of a lithographic printing plate using the same radiation sensitive composition as in U.S. Pat. No. 5,372,915. But in this related patent the radiation sensitive composition is imagewise exposed to activating radiation, and then the printing plate is heated to provide reduced solubility in exposed areas and increased solubility in unexposed areas. The unexposed areas of the printing plate are then removed with an aqueous alkaline developing solution. Although the composition is the same, a positive or a negative lithographic image is produced in each respective patent by varying the activating radiation and adding a blanket heating step.
PCT/GB95/02774 is an example of forming a negative lithographic image from a positive working photosensitive composition comprising a naphthoquinone diazide ester and a phenolic resin. In the disclosed method the photosensitive composition is first uniformly exposed to ultraviolet radiation to render the composition developable. The plate is then imaged with an infrared laser to insolubilize the imaged areas. Those areas not exposed by the laser are then removed with a developer.
While advances have been made to provide negative working printing plates with infrared laser radiation, there continues to be a need for a simplified process to manufacture long-run positive working lithographic printing plates.